In a large-scale architectural structures, such as buildings, room cooling is carried out by producing cold water using a refrigerator and circulating the cold water through the piping installed inside the building, and cooling the room by heat exchange with the room air.
An example of evaporators provided in a refrigerator is shown in FIG. 6. Such an evaporator is comprised of a cylindrical container 1 for admitting a cooling medium, and containing numerous heat conducting tubes 2 bundled in a zigzag fashion for flowing the cold water.
Heat exchanger tubes 2 are divided into a tube group-a communicating with a cold water entry opening 3, two tube groups-b, -c communicating the water chambers (omitted in the diagram) provided on each end of the container 1, and a tube group-d communicating with a cold water exit opening 4 (equal number of tubes are provided in each group), so that the cold water flowing from the cold water entry opening 3 flows through the tube group-a to reach one water chamber and then reverses the flow through the tube group-b to enter other water chamber and reverses the flow for the second time through the tube group-c to reach the other water chamber and reverses the flow for the third time through the tube group-d to discharge from the cold water exit opening 4. The cold water flows through the tube groups and travels the distance of the container 1 twice, and in the process, it exchanges heat with the cooling medium and is cooled by the cooling medium introduced into the container 1 through a different path. On the other hand, the cooling medium are heated by the cold water to boil and vaporize.
However, the evaporators having such a structure present the following problems.
(1) In the conventional evaporators, the number of heat exchanger tubes within each tube group is the same, and the tube lengths are also the same. However, when those tubes in the upstream environment in the flow direction are compared with those tubes in the downstream environment, the flow speed of the cold water flowing in the tubes is almost constant, but the temperature differential between the cold water flowing inside the tube and the cooling medium flowing on the outside of the tubes is small in the downstream environment so that the heat flux is less compared with the heat flux in the upstream environment, resulting in reducing the rate of heat conduction in the downstream tube groups.
(2) In the upstream tube groups, there is a temperature difference between the cold water flowing in the tube and the cooling medium flowing around the outside of the tube, and the heat conduction flux is larger compared with the heat conduction flux in the upstream side, thereby increasing the heat conduction rate. Although increased heat conduction rate is not a problem in itself, cooling medium is actively vaporized in the vicinity of the upstream tubes to an extent to increase the void factor to impede heat exchange between the liquid phase of the cooling medium and cold water, resulting that heat conduction rate is reduced even in the upstream tube groups.
(3) In the upstream tube groups, the vapor/liquid interface (frost level, or more accurately, an interface between the vapors of the cooling medium and a vapor/liquid two-phase mixture) is raised, while in the downstream tube groups, affected by the rise in the vapor/liquid interface in the upstream tube groups, the vapor/liquid interface is lowered Therefore, if the height of the heat exchanger tubes in the uppermost stage of each tube group are the same, the heat exchanger tubes in the uppermost stage of the downstream tube groups are exposed to vapor phase of the cooling medium, thereby impeding heat exchange between the cooling medium and the cold water, to result in reducing the heat conduction rate even in the downstream tube groups.